固体地球物理学术报告通知-盛一笑

We revisit velocity changes caused by the 2004 Parkfield earthquake through the analysis of the coda of repeating earthquakes. Our results reveal the change to be strongly frequency-dependent. Low-frequency components of the coda of repeating earthquakes are more affected by the velocity alteration than high-frequency components. We interpret this to indicate that the coseismic velocity reduction primarily occurs at a shallow depth, and is expressed in low-frequency energy resulting from reverberations near the Earth’s surface. This can only be observed at low-frequencies because the shallow crust is highly dissipative. The high-frequency component is relatively unaffected as it is comprised of body waves scattered throughout the crust. We support this argument with direct observations of seismic wavefields across a wide range of depths in the shallow crust, supplemented by using downhole geophones and distributed acoustic sensing (DAS) measurements.